What you really need to is ditch windows for Linux if you want to mine with that many cards on a single rig. Windows is unstable and tends to blow things up with random windows updates. Linux is so much more stable and is much easier to get a rig with that many cards running.
I hear you. Is it possible to easily change core and memory voltage in Linux? I was had the impression you couldn't, but I might be mistaken. That has been the reason I have stayed with Windows.
Also from the Bios I have not been able to change the core voltage as much as I would have wanted. Not sure if I did something wrong.